The penicillin focus. I. Distribution of potential at the cortical surface

The potential field of a penicillin focus of controlled size was recorded from a rectangular array of 12 electrodes occupying a 4 X 6 mm area on the exposed anterior sigmoid gyrus of the cat. The array was made with 2 mm interelectrode spacing of 0.4 mm I.D. glass capillary tubes filled with Ringer's in agar, excepting one containing penicillin to create the focus. Early in the development of the focus a negative spike appears at the penicillin electrode reaching amplitudes of as high as 3,000 muV while all of the other electrodes showed no synchronous activity of more than 100 muV. Within the first 15 min spike activity becomes visible at about 100 muV at all the electrodes and after 30 min, waveforms of all activity become quite stable. At the penicillin-containing electrode, at intervals of 2--10 sec each, a negative sharp wave of up to 4 mV occurs, having a shorter rise time than fall time. A smaller positive spike of about 30 msec duration which showed marked variations in amplitude often preceded the stable negative sharp wave. During the rise of the negativity at the penicillin electrode, practically all the surrounding electrodes showed a predominantly positive spike. This was occasionally followed by negativity during the falling phase at the penicillin electrode. Displays of potential surfaces interpolating the average values at the 12 recording points on the cortex at 4 msec intervals demonstrate a relationship between the field of the sharp wave at the penicillin focus which is less than 2 mm in diameter and that of the surrounding electrode which indicates that symmetrically located synaptic inhibitory processes are strongly activated in areas adjacent to the small simultaneous excitation at the penicillin electrode. Degrees of attenuation of interictal spikes that take place between cortex and scalp are estimated from the measured potential distributions. Explanation is offered for the reported apparent discrepancies between the findings at the cortex and the scalp EEG.